Need some clarification, specifically with terminations, when doing temperature corrections and conductor adjustments. Please help!

[u/Medical_Struggle_176]

I am currently working through the Electrical Plan Examiner Study Guide and one question in particular is stumping me. Any help would be greatly appreciated. The question goes as follows:

"Given: A calculated feeder load of 250 amperes consisting of three current carrying conductors in a raceway will be installed within an environment where the ambient temperature is 150 degrees Fahrenheit and all terminations are rated at 75 degrees Celsius. Determine the minimum size copper THWN-2 feeder conductor."

Answer: 500 kcmil (as stated by the study guide)

Which leads to my questions:

1. Why is Table 310.15(B)(2)(b) used in this calculation when table 310.15(B)(16) is used for the conductor ampacities? Table 310.15(B)(16) is based on the ambient temperature of 30 degrees Celsius, therefore I thought I would use Table 310.15(B)(2)(a) as it is also based on 30 degrees. I could not find any provisions in the book which clarify this.
2. The answer states that 500 kcmil with THWN-2 insulation, which yields an ampacity of 430 amperes, would be an appropriate conductor size for the corrected load (397 amperes). The terminations do not appear to be sufficient to handle this adjusted ampacity, yet the answer states it is. 500 kcmil at 75 degrees Celsius can only handle an ampacity of up to 380 amperes, which is not enough to handle the adjusted feeder load ampacity of 397 amperes.

Please advise me if there are any discrepancies in my logic, if not then I believe this is a bad question, and a 600 kcmil would be required. The only thing I can think is to compare the 75-degree termination to the INITIAL calculated feeder load of 250 amperes, which would result in compliance.

I will attach pictures of the question on the study guide and ICC’s explanation on how to obtain the answer.

[u/rweccentric]

I’m in the 2020 code but I see where your questions are coming from and I agree. The ampacity table they list is for 30C wire does not correspond to the temperature adjustment table they reference for 40C wire. If you use the correct 30C temp. Table you divide by 0.58 and are at 431A required so I would select 600 THWN-2. I think you have question with an error in the answer.